A. Field of Invention
The present invention relates to heavy metal fluoride glasses for improved performance either when doped with a lanthanide series rare earth and thereafter formed into an optical fiber for use in amplification and other purposes, or which can be utilized as a bulk glass as an improvement over conventional ZBLAN glass.
B. Description of Related Art
Many glasses of varying compositions have been disclosed and discussed as a basis for utilization as a host, such material either used as a core for an optical fiber to be doped with a lanthanide series heavy metal, or for use as a bulk glass. Thus, for many years the standard glass was that known by the acronym ZBLAN, which has become a classic composition whose formula, in terms of mole percentage, is 50-55 ZrF.sub.4 --16-24 BaF.sub.2 --3-5 LaF.sub.3 --2-4 AlF.sub.3 --16-24 NaF. ZBLAN has found wide acceptance as a glass from which fluoride fibers may be formed. In fact, when such fluoride fibers are doped with a lanthanide series metal, such as praseodymium, those fibers, when used in optical systems for their amplification properties, have become referred to as PDFFA's, which stands for praseodymium-doped fluoride fiber amplifiers.
Among the patents which refer to such fibers and the mechanism by means of which they perform to produce an optical gain in an optical fiber system, is U.S. Pat. No. 5,309,452, which is specifically directed to a praseodymium laser system. However, as pointed out in that patent, research has been done into improvements in ZBLAN glass. Thus, where specific characteristics are desired to be incorporated into the ZBLAN, or simply as a less expensive substitute, zirconium ions can be replaced with hafnium and/or thorium ions, and sodium ions can be replaced with potassium, rubidium, cesium, or thallium ions. As a consequence, much experimentation based on the original ZBLAN has been performed in order to derive glasses having specific characteristics.
While for the specific purpose of forming a PDFFA glass, many different elements can be added, for example, sensitizers such as rare earth ions, including erbium, ytterbium, and neodymium ions, with additional sensitizers such as chromium ions, the present state of the art has been notably deficient in recognizing the importance of indium, and specifically indium trifluoride, as an ingredient of a glass to be used as a host for PDFFA fibers and as a bulk glass.
Indium fluoride has been mentioned in compositions disclosed in a publication of S. T. Davey, D. Szebesta and R. Wyatt, Properties of Novel Glasses for the Optimization of Pr-doped Fluoride Fibre Amplifiers, presented at the SPIE International Symposium on Fibre Laser Sources and Amplifiers V, Boston, Mass., September, 1993. In this publication the authors suggest the replacement of AlF.sub.3 in traditional ZBLAN with YF.sub.3 and InF.sub.3, as well as the partial replacement of NaF with CsF for improved stability. The publication avers that with such glass the .sup.1 G.sub.4 lifetime of Pr.sup.3+ is almost 25% longer than in classic ZBLAN. It also refers to a publication of Ohishi et al. that improved PDFFA performance is possible from an InF.sub.3 -based glass. Nevertheless, the real importance of the use of a significant amount of indium fluoride has not been recognized in the prior art.
It is, therefore, an object to the present invention to provide a heavy metal fluoride glass which is based on the use of indium fluoride in a significant amount not heretofore contemplated, indeed, a glass in which the InF.sub.3 is used in an amount of 10 to 40 mole percent, and preferably in the upper range of that amount. The use of such a quantity of InF.sub.3 far exceeds that suggested by the Davey et al. publication, in which the AlF.sub.3 in ZBLAN, has been replaced with YF.sub.3 and InF.sub.3. For example, if the InF.sub.3 were utilized completely to replace the AlF.sub.3 without the use of YF.sub.3, the mole percentage of InF.sub.3 would be that of the AlF.sub.3 in a classical ZBLAN composition, in which 4 mole percent of AlF.sub.3 is typically used. Yet, such a minor variation does not even approximate that of the use of InF.sub.3 as disclosed in the present invention, where at least 10 mole percent of InF.sub.3, and often up to 40%, is utilized, and therefore, greatly improved transmission characteristics of, e.g., a praseodymium-doped glass, are achieved.
As a consequence, it is another object of the present invention to devise a glass in which a high NA, low-loss composition can be used either in bulk or as a PDFFA fiber amplifier with superior optical properties, and which glass is based on the use of InF.sub.3 as the principal element, usually in an amount of about 25 mole percent of the entire composition. It is another object of the present invention to devise an optical fiber doped with praseodymium, which fiber is formed from a glass having significantly improved transmission characteristics and marked by the use of at least 10% InF.sub.3.